(a) Technical Field of the Invention
The present invention relates generally to a protection circuit for a lithium battery pack that serves as a power source of a power tool, and in particular to a battery protection circuit that, as compared to the conventional battery protection circuit, features simple structure, low costs, being easy to match currently existing depressing-controlled circuit of power tools, and functions detect battery voltage and temperature in the discharging process of the battery for immediately cutting off the discharge current when abnormal events occur.
(b) Description of the Prior Art
Lithium ion batteries, lithium polymer batteries, and other lithium based chemical cells have generally different discharging characteristics from other types of secondary cells. The lithium based batteries or cells are easily subject to damage when they are over-discharged or being of over-temperature. Further, over-temperature may also cause explosion of the lithium based batteries, especially when a number of lithium batteries are connected in series and/or in parallel to effect high current charging and discharging for power tools that requires much larger output power than a single cell can provide. In such applications, the lithium batteries are much more easily subject to damages caused by over-discharging and the costs are much higher when the batteries are so damaged. Also, explosion of the batteries is more powerful, if it happens. Apparently, a protection circuit is often employed to detect voltage and temperature of the lithium battery during the operation thereof and to immediately cut off the discharge current at the time when abnormal events occur. Such a protection circuit must also ensure little leakage current when a power tool having such as protection circuit is put in non-operation condition. However, when a protection circuit is incorporated in a battery pack, it often mismatches a reversal/forward rotation circuit and a speed variation circuit that are commonly present in the power tool.
A regular nickel-cadmium battery or nickel-metal-hydride battery is taken as an example to explain the circuit of a power tool that is a combination of a reversal/forward rotation circuit, a speed variation circuit, and a motor of the power tool, and the battery. FIG. 1 of the attached drawings illustrate the example circuit, which comprises a reversal/forward rotation circuit 10 having a pair of single-pole-double-throw switches 11, 12, which include externally-located control members for switching the switches 11, 12. The two throw terminals of the two switches 11, 12 are coupled to each other respectively and are connected to terminals of a motor 30 respectively. A common terminal of the single-pole-double-throw switch 11 is connected to an anode of a battery 40, while a common terminal of the single-pole-double-throw switch 12 is connected to a third throw terminal of a single-pole-triple-throw switch 21 that constitutes in part a speed variation circuit 20.
Besides the single-pole-triple-throw switch 21, the speed variation circuit 20 also comprises a pulse width modulation (PWM) generation circuit 22, a power transistor 23, a switch 24 that is closed by the switching operation of the single-pole-triple-throw switch 21, and a diode 25. The switch 21 has a common terminal connected to a cathode of the battery 40, a first throw terminal that is open-circuited, and a second throw terminal that is connected to a grounding terminal of the PWM generation circuit 22 and also connected to an emitter of the power transistor 23. The third throw terminal, which as mentioned above, is connected to the common terminal of the single-pole-double-throw switch 12. The PWM generation circuit 22 has an input terminal that is connected to the anode of the battery 40 and an output terminal connected to a base of the power transistor 23. A collector of the power transistor 23 is connected to the third throw terminal of the switch 21. The switch 24 has a terminal connected to the anode of the battery 40 and an opposite terminal connected to the third throw terminal of the switch 21 so that the switch 24 is induced to close by the operation of switching the switch 21 to the first throw terminal. The diode 25 has a negative terminal connected to the anode of the battery 40, and a positive terminal connected to the third throw terminal of the switch 21.
When an operation button or actuation button is manually depressed, the switch 21 is driven away from the first throw terminal and thus opening the switch 24. When the switch 21 reaches the second throw terminal thereof, the emitter of the power transistor 23 is put in electrical connection with the cathode of the battery 40 and the PWM generation circuit 22 is actuated to supply a PWM signal from the output terminal thereof to the base of the power transistor 23. The tool operation button, when depressed or actuated, varies a variable resistor, in such a way that the extent that the operation button is depressed corresponds to the PWM signal ratio, and thus, the rotational speed and output torque of the motor 30 are increased in accordance therewith. When the operation button is depressed to the bottom most position, the switch 21 is driven to the third throw terminal and the power transistor 23 is bypassed. In this condition, the motor 30 is directly driven by the closed third throw terminal of the switch 21. When the operation button is no longer depressed and is thus released, the switch 21 is biased back to the first throw terminal and power supplied to the motor 30 is cut off and the switch 24 is controlled by such a backward switching operation to close, which applies a positive voltage from the battery 40 to both terminals of the motor 30 and thus inducing braking operation of the motor 30. This stops the operation of the motor 30. Further, since, in this situation, the switch 21 breaks the connection between the cathode of the battery 40 and the power transistor 23 and the grounding terminal of the PWM generation circuit 22, the leakage current is reduced.
As shown in FIG. 2, by incorporating the above discussed reversal/forward rotation circuit 10 and the speed variation circuit 20 in a power tool that is powered by a lithium battery pack 70, together with a protection circuit 50 arranged within the battery pack 70, to effect cutting off the discharge current at the time when abnormal events occur, a discharge circuit of a lithium battery set 71, which is a circuit that is in connection with the previously discussed switch 21, must be connected in series with a power transistor MOS1. Such an arrangement, when incorporated to the reversal/forward rotation circuit 10 and the speed variation circuit 20, raises a problem where the power supplied to the PWM generation circuit 22 will be cut off by the power transistor MOS1 and the PWM generation circuit 22 fails to properly function.
An example solution to such a problem is illustrated in FIG. 3, wherein the lithium battery pack 70 is further provided with two transmission lines, of which one is branched off the connection between the cathode of the lithium battery pack 71 and the speed variation circuit 20, which line forms a terminal that provides an electrical signal labeled V- , the power transistor MOS1 being arranged on the line, and the other line is coming from a terminal of the protection circuit 50 that receives a positive electrical signal labeled VDD, a switch 51 being arranged on the line. If necessary, a temperature detection terminal Vth can be further added for detecting the temperature during a charging process. When the switch 21 is slightly depressed, the switch 51 is closed to provide VDD to drive the protection circuit 50 and conducting on the power transistor MOS1 to have the common terminal of the switch 21 connected to the anode of the battery pack 70 to thereby form an operation circuit.
The known circuit, however, suffers insufficient space inside a power tool for arrangement of the switching circuit. In other words, the power tool has to be redesigned with an increased size. In addition, two power transistors connected in series make efficiency low and costs increased.
The present invention is aimed to solve the drawbacks of the conventional protection circuit employed in power tool that uses a lithium battery pack as a power source.